In the construction, domestic appliance and automotive industries, metal substrates are increasingly being used, and increased demands are being made of such substrates in terms of corrosion protection. In addition to known hot galvanized or electrolytically galvanized steel sheets, coated surfaces with zinc, aluminum alloys are also used. Other metals, especially aluminum substrates or aluminum alloys, may also be employed. There is an extensive prior art relating to the deposition of anticorrosive layers on metal surfaces. In the prior art, substantially aqueous solutions, which may optionally also contain organic polymers, are applied to the surface. The term conversion treatment indicates that the conversion solution reacts chemically with the metal surface during the treatment, with the formation of an anticorrosive layer, which may contain metal atoms of the substrate and optionally polymers.
It is known that the use of chromium compounds or the addition of chromate to the treatment solution results in a substantially better anticorrosive layer and also in better adhesion. In principle, however, it is preferable, because of the physiologically harmful effect of chromium, to avoid using this heavy metal in such processes. Furthermore, attempts are also being made to reduce other heavy metals, for example cobalt, copper, nickel, as far as possible. The chromium-free conversion treatment of metal surfaces with fluorides of boron, silicon, hafnium, titanium or zirconium in conjunction with organic polymers to produce a conversion layer is known. The pretreatment solutions are applied either by the rinse process or by the no-rinse process. In the rinse process, the conversion layer is rinsed after it has formed; in the no-rinse process, the solution is applied and dried without rinsing.
Accordingly, DE-C-24 33 704, for example, describes treatment baths that may comprise polyacrylic acid or its salts as well as ammonium fluorozirconate. The conversion layers that form are said to exhibit improved adhesion of the lacquer to subsequent layers. DE-A-197 51 153 discloses a chromate-free coating for coil-coating steel sheets, in which titanium, manganese and/or zirconium salts of olefinically unsaturated polymerizable carboxylic acids and further olefinic monomers are applied together with initiators and are subsequently crosslinked by UV radiation. In both cases, further lacquer layers can subsequently be applied.
WO 01/85853 describes a method of coating metal sheets with a chromate-free UV-curing coating agent that comprises polyester acrylates and polyurethane acrylates and that forms an anticorrosive layer on the substrate after crosslinking.
WO 01/32321 describes a method of coating metal substrates wherein a second coating agent is applied to a first coating agent after curing, the second coating agent being curable by radiation. Before being coated with the coating agents, the metal substrate is treated with a conventional inorganic pretreatment solution in order to produce a conversion layer.
WO 02/24344 describes a method for the multi-layer lacquering of metal surfaces in which at least one lacquer-like layer is applied to the substrate, wherein either the substrate is provided beforehand with an anticorrosive layer or no anticorrosive layer is used. The anticorrosive layer and the lacquer-like layer are both crosslinked, UV-crosslinking binders together with photoinitiators being used for the lacquer-like layer. The lacquer-like layer may optionally additionally also comprise corrosion inhibitors and/or conductive particles.
A problem with the use of conversion protective layers and UV-curing coating agents is the adhesion of the coating agent to the metal substrate. In the case of chromate-free pretreatments in particular, adhesion is often poor. In addition, even slight defects in the adhesion or cracks in the coating lead to increased corrosive attack on the underlying metal substrate. Such cracks and defects of adhesion can occur, for example, directly in the process of curing by actinic radiation as a result of shrinkage, or when the metal substrates are mechanically formed. Difficulties in terms of adhesion are encountered especially when the corresponding substrates are in the form of sheets or strips and are to be protected against corrosion and coated in that form, because such strips or sheets must subsequently be cut and brought into the appropriate later form. A further problem lies in the coating of coil-coating materials themselves because, due to the process, only very short reaction times are available in this application. It is furthermore desirable to keep the number of successive method steps as low as possible. The pretreatment solutions are generally acidic solutions, so that the adhesion problems cannot simply be solved by the addition of known adhesion agents, for example silane compounds.